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Ferroelectric control of the spin texture in GeTe

Spin-orbit coupling effects in materials with broken inversion symmetry are responsible for peculiar spin textures, giving rise to intriguing phenomena such as intrinsic spin Hall effect. Among these materials, ferroelectrics allow for non-volatile control of the spin degree of freedom through the electrical inversion of the spin texture, based on their reversible spontaneous polarization. Finding suitable ferroelectric semiconductors would be a fundamental achievement towards the implementation of novel electronic and spintronic devices combining memory and computing functionalities.
Germanium Telluride emerges as promising candidate, since theoretically proposed as the father compound of the new class of ferroelectric Rashba semiconductors. Its ferroelectricity provides a non-volatile state variable able to generate and drive a giant bulk Rashba-type spin splitting of the electronic bands. Its semiconductivity and silicon-compatibility allows for the realization of spin-based non-volatile transistors.
A European team of both experimentalists and theoreticians from Italy (Politecnico di Milano, IFN-CNR, CNR-SPIN, CNR-IOM) and Germany (Paul-Drude-Institut für Festkörperelektronik, Universität Würzburg) has demonstrated the ferroelectric control of the Rashba spin texture in GeTe probed by spin and angular resolved photoemission spectroscopy at the Advanced Photoelectric Effect experiments (APE) beamline and supported by NFFA.
High-quality GeTe film were grown by molecular beam epitaxy at the Paul-Drude-Institut and protected by a capping layer of tellurium. By proper surface preparation strategies, we were able to obtain GeTe films with either Ge- or Te- surface terminations, as checked by X-ray Photoemission spectroscopy performed at the high-energy branch of APE. The two surfaces were found to display opposite uniform remanence ferroelectric states, as measured by Piezoresponse Force Microscopy (PFM) at Polifab, the reference facility of Politecnico di Milano for micro and nanofabrication. Spin- and Angle-Resolved Photoemission Spectroscopy (SARPES) carried out at the low-energy branch of the APE beamline was used to access the band structure of GeTe for the two different surface terminations (i.e. ferroelectric polarizations). Density functional theory calculations (CNR-SPIN) are able to get the main features of the real band structure for the two GeTe surfaces. The investigation was able to demonstrate the inversion of the spin population of each Rashba sub-band with the remanent ferroelectric polarization (Figure 1).

Figure 1. (a, a’) PFM ferroelectric hysteresis loops and the pristine polarization states for the as-prepared Te- and Ge-terminated GeTe(111) surfaces, respectively. (b, b’) DFT calculations of the k-resolved spin polarization along two high symmetry crystallographic directions. The main bulk Rashba bands are marked as B1 and B2. The black dashed line indicates the wave vector k of SARPES measurements. (c, c’) Spin-polarized currents and spin asymmetries (Px) versus binding energy at the wave vector k. The peaks correspond to the intersection of the Rashba bands B1 and B2 with the vertical dashed line at k. (d, d’) Constant energy maps for the Te- and Ge-terminated surfaces. Blue and red arrows indicate the sense of circulation of spins, opposite for the two opposite ferroelectric polarizations.

To exploit the concept of reconfigurable spin functionality in GeTe, we demonstrated the crafting of non-volatile ferroelectric patterns in GeTe films at the nanoscale by using the tip of an atomic force microscope. In this way, we opened the route towards devices with engineered spin configurations and to the non-volatile electric control of spin transport in a silicon-compatible semiconductor.

Figure 2. The spin texture in ferroelectric Rashba semiconductors can be controlled by reversing their remanent polarization. The illustration (Cover of Nano Letters, May 2018) shows the local manipulation of the spins in Rashba bands obtained patterning ferroelectric nanodomains with the conductive tip of an atomic force microscope.

The result received the recognition of Nano Letters and deserved the Cover of the May 2018 issue of the journal (Figure 2). The exploitation of ferroelectric Rashba semiconductors is supported by Fondazione Cariplo and Regione Lombardia through the project ECOS (Electric Control Of Spin transport in ferroelectric Rashba semiconductors), grant n° 2017-1622.